1. Field of the Invention
Embodiments disclosed herein relate generally to cutting elements containing a layer of ultrahard material. More particularly, embodiments of the present disclosure relate to cutter assemblies which include such cutting elements for use in a drill bit or other downhole cutting tool.
2. Background Art
Polycrystalline diamond compact (“PDC”) cutters have been used in industrial applications including rock drilling and metal machining for many years. In a typical application, a compact of polycrystalline diamond (PCD) (or other ultrahard material) is bonded to a substrate material, which is typically a sintered metal-carbide to form a cutting structure. PCD comprises a polycrystalline mass of diamonds (typically synthetic) that are bonded together to form an integral, tough, high-strength mass or lattice. The resulting PCD structure produces enhanced properties of wear resistance and hardness, making PCD materials extremely useful in aggressive wear and cutting applications where high levels of wear resistance and hardness are desired.
A PDC cutter may be formed by placing a sintered carbide substrate into the container of a press. A mixture of diamond grains or diamond grains and catalyst binder is placed atop the substrate and treated under high pressure, high temperature conditions. In doing so, metal binder (often cobalt) migrates from the substrate and passes through the diamond grains to promote intergrowth between the diamond grains. As a result, the diamond grains become bonded to each other to form the diamond layer, and the diamond layer is in turn integrally bonded to the substrate. The substrate often comprises a metal-carbide composite material, such as tungsten carbide-cobalt. The deposited diamond layer is often referred to as the “diamond table” or “abrasive layer.”
Typically, the substrate used to form the cutting element is chosen based upon properties which are beneficial to forming the abrasive layer of ultrahard material on the surface of the substrate using a high pressure/high temperature (HPHT) process, for example, the type and quantity of metal binder (e.g., cobalt) in the substrate and the grain size of the metal carbide used to form the substrate. However, substrates which have properties beneficial to the formation of the abrasive layer may not have optimum properties with respect to operating conditions, for example sufficient erosion resistance, corrosion resistance, hardness, toughness, braze strength, etc. As a result, cutting elements may be retrieved from a previously used tool (e.g., drill bit) which has been used to cut an earthen formation and which could otherwise be re-used in another tool (e.g., either a new or rebuilt drill bit) except for the damage to the cutting element. Such cutting elements are typically discarded at significant cost (as an otherwise useable abrasive layer can no longer be utilized in another drill bit).
Another significant factor in determining the longevity of PDC cutters is the exposure of the cutter to heat. Exposure to heat can cause thermal damage to the diamond table and eventually result in the formation of cracks (due to differences in thermal expansion coefficients) which can lead to spelling of the polycrystalline diamond layer, delamination between the polycrystalline diamond and substrate, and conversion of the diamond back into graphite causing rapid abrasive wear. The thermal operating range of conventional PDC cutters is typically 700-750 ° C. or less.
As mentioned, conventional polycrystalline diamond is stable at temperatures of up to 700-750° C. in air, above which observed increases in temperature may result in permanent damage to and structural failure of polycrystalline diamond. This deterioration in polycrystalline diamond is due to the significant difference in the coefficient of thermal expansion of the binder material, cobalt, as compared to diamond. Upon heating of polycrystalline diamond, the cobalt and the diamond lattice will expand at different rates, which may cause cracks to form in the diamond lattice structure and result in deterioration of the polycrystalline diamond. Damage may also be due to graphite formation at diamond-diamond necks leading to loss of microstructural integrity and strength loss, at extremely high temperatures.
Cutters are conventionally attached to a drill bit or other downhole tool by a brazing process. In the brazing process, a braze material is positioned between the cutter and the cutter pocket. The material is melted and, upon subsequent solidification, bonds (attaches) the cutter in the cutter pocket. Selection of braze materials depends on their respective melting temperatures, as higher braze temperatures cannot be used without resulting in damage to the diamond layer prior to the bit (and cutter) even being used in a drilling operation.
This temperature restriction greatly limits the number of alloys that can be used as braze alloy for cutting elements with diamond layers thereon because most brazing alloys that provide sufficient shear strength for bonding cutting elements to a drill bit also require brazing at temperatures above 700° C. Therefore, alloys suitable for brazing cutting elements with diamond layers thereon have been limited to only a couple of alloys which offer low enough brazing temperatures to avoid damage to the diamond layer and high enough braze strength to retain cutting elements on drill bits. Further, in most manual brazing processes, it is difficult to control the brazing temperature.
Accordingly, there exists a continuing need to develop ways to extend the life of a cutting element and for developments in cutting element attachment methods to prevent thermal damage to PDC cutters during installation in a downhole tool and improve the ease of rebuilding such downhole tools.